A fuel cell is a power generation system for producing electrical energy through an electrochemical redox reaction of an oxidant and hydrogen in a hydrocarbon-based material such as methanol, ethanol, or natural gas.
Typical examples of fuel cells are polymer electrolyte membrane fuel cells (PEMFC) and direct oxidation fuel cells (DOFC). A direct oxidation fuel cell that uses methanol as a fuel is called a direct methanol fuel cell (DMFC). The polymer electrolyte membrane fuel cell is an environmentally-friendly energy source that can replace fossil fuel energy. It has several advantages such as high power output density, high energy conversion efficiency, operability at room temperature, and the capability to be down-sized and tightly sealed. Therefore, it can be widely applied to various areas such as non-polluting automobiles, residential electricity generation systems, and as portable power sources for mobile communication equipment and military equipment.
The polymer electrolyte membrane fuel cell has an advantage of having high energy density, but it also has the problems of requiring careful handling of hydrogen gas, or requiring accessory facilities such as a fuel-reforming processor for reforming a fuel gas such as methane, methanol, or natural gas to produce the hydrogen required.
In contrast, a direct oxidation fuel cell generally has lower energy density than that of a polymer electrolyte fuel cell, but it has the advantages of easy handling of the liquid-type fuel, operability at low temperatures, and does not require additional fuel-reforming processors. Therefore, such direct oxidation fuel cells may be appropriate systems for small-scale and general purpose portable power sources.
It is also highlighted as a novel portable power source because it has from four to ten times higher energy density than that of small lithium batteries.
The fuel cell has a stack formed by stacking several to a plurality of unit cells in multi-layers, which generates electricity. Here, each unit cell is made up of a membrane-electrode assembly (MEA) and a separator (also referred to as a bipolar plate).
The membrane-electrode assembly has an anode (referred to as a fuel electrode or an oxidation electrode) and a cathode (referred to as an air electrode or a reduction electrode) separated from each other by a polymer electrolyte membrane.
As for the polymer electrolyte membrane, research on a polystyrene sulfonic acid-based polymer resin has been actively performed since its initial development stage in the 1960s. In 1968, E. I. Dupont de Nemors, Inc. developed a perfluorinated sulfonic acid-based cation exchange resin (product name: NAFION®), which is reported to have much improved proton conductivity, electrochemical stability, and so on. However, since then, research has been more widely focused on the practical use of a fuel cell using NAFION®. NAFION® has hydrophobic polytetrafluoroethylene as a main chain and a functional group including a hydrophilic sulfone group at its side chain. On the other hand, a fluorine-based cation exchange resin with a similar structure to NAFION® has been developed by Asahi Chemical, Asahi Glass, Tokuyama Soda, and so on.
However, a NAFION® polymer electrolyte membrane, which has already become commercially available, has many more advantages than a hydrocarbon-based polymer electrolyte membrane in terms of oxygen solubility, electrochemical stability, durability, and the like. Since the NAFION® polymer electrolyte membrane appears to be conductive to hydrogen ions when about 20% of the polymer weight therein becomes hydrated (i.e. a sulfone group included in a pendant group is hydrolyzed into a sulfonic acid), a reaction gas used in a fuel cell must be saturated by water to hydrate the electrode membrane. However, the water gradually evaporates above its boiling point of 100° C., and accordingly the resistance of the polymer electrolyte membrane increases, deteriorating cell performance. In addition, the NAFION® polymer electrolyte membrane, which is commonly 50 to 175 μm thick, can be increased or decreased in thickness to improve the dimensional stability and mechanical properties of a fuel cell. However, when the thickness is increased, the conductivity of the polymer electrolyte membrane decreases, and when it is decreased, the mechanical properties deteriorate. When the polymer electrolyte membrane is used in a methanol fuel cell, non-reacted liquid methanol fuel passes therethrough during the cell operation (i.e., methanol crossover), thereby deteriorating cell performance as well as causing a fuel loss, because the methanol is oxidized at a cathode.
Therefore, various methods for preventing the methanol from crossing through the polymer electrolyte membrane have been recently researched and suggested. For example, a method of sputtering palladium on the surface of a polymer electrolyte membrane or forming a thin polymer layer with high resistance against methanol transmission thereon by plasma polymerization has been reported. Another method, of forming nano-sized silica (SiO2) on the polymer electrolyte membrane in a sol-gel method, has also been revealed.
However, the modified polymer electrolyte membranes produced by using a sputtering and plasma method are insufficiently competitive in price. The sol-gel method of forming silica also has a problem of low productivity since it needs a great deal of washing to prevent silica from being poisoned by Cl− ions due to the reaction of the precursor of silica (tetraethoxyorthosilicate) with hydrochloric acid.